I would guess the free 'demo' version would open them if they were Eagle files but .sch and .brd are used by other CAD programs as well. Kicad uses them: https://kicad-pcb.org/ and I think some versions of Protel do too.
If I recall correctly, the free version of eagle could open documents made by the pro version, but any changes you wanted to make would be bound by the free version rules. So you can't edit the inner copper layers, or place components on multiple sheets, or move layout features outside the free version boundaries.